Rule out cheerful cook (4)
I believe the answer is:
chef
'cook' is the definition.
(both can mean someone who cooks)
'rule out cheerful' is the wordplay.
'out' suggests deleting specific letters.
'cheerful' with 'rule' taken out is 'CHEF'.
(Other definitions for chef that I've seen before include "Professional cook" , "Chief male cook in restaurant" , "Cooker?" , "Originator of courses" , "One who prepares courses" .)